The available techniques for the measurement of residual stress mainly include hole drilling and X-Ray diffraction. The measurement by hole drilling provides a higher accuracy, which is premised on damaged surface status of materials. The surface needs to be prepared by polishing treatment before the measurement, producing residual stress undoubtedly. Also the process of sticking strain gages is time-consuming so that a real-time detection cannot be achieved. X-Ray diffraction is mainly applied in non-destructive measurement. It is not possible to realize an on-site measurement with X-Ray diffraction due to its harmful radiation to human body, as well as long testing time, even though it has been put on the market.
Although many studies have been carried out on stress measurement by using ultrasound, it cannot meet the requirement of on-site measurement due to unreasonable design of structure, various interfering factors, and poor repeatability. The critically refracted longitudinal (LCR) wave, which propagates along the surface of the specimen, is not sensitive to the roughness of the surface. Therefore there is no need to conduct special treatments to the surface, and it is capable of measuring the average value of stress value at a depth of about one wavelength from a surface based on a simple principle of measurement.